fix(Slugged+History+Scoped): fix SQL errors when using the 3 plugins together

lib/friendly_id/history.rb

@@ -108,11 +108,14 @@ def slug_history_clause(id)
     def scope_for_slug_generator
       relation = super
       return relation if new_record?
-      relation = relation.merge(Slug.where('sluggable_id <> ?', id))
+
+      # joins(:slugs) is needed but will be added to the relation by `exists_by_friendly_id?`
+      sw = Slug.where('sluggable_id <> ?', id)
       if friendly_id_config.uses?(:scoped)
-        relation = relation.where(:scope => serialized_scope)
+        # scope is in Slug not in the model table
+        sw = sw.where(:scope => serialized_scope)
       end
-      relation
+      relation.merge(sw)
     end
 
     def create_slug

test/history_test.rb

@@ -204,6 +204,44 @@ class Restaurant < ActiveRecord::Base
   end
 end
 
+class HistoryTestWithSlugScopeAutomaticRegen < HistoryTest
+
+  class Restaurant < ActiveRecord::Base
+    extend FriendlyId
+    belongs_to :city
+    friendly_id :name, :use => [:slugged, :scoped, :history], :scope => :city
+
+    def should_generate_new_friendly_id?
+      slug.blank? or name_changed?
+    end
+  end
+
+  def model_class
+    Restaurant
+  end
+
+  test 'should allow reversion back to a previously used slug and recreation based on name when blank' do
+    city = City.create!
+    with_instance_of(model_class, name: "My #{rand(1000)} name", city_id: city.id) do |record|
+      record.name = 'bar'
+      record.save!
+      assert_equal 'bar', record.friendly_id
+      record.name = 'foo'
+      record.save!
+      assert_equal 'foo', record.friendly_id
+
+      record.slug = 'something-else'
+      record.save!
+      assert_equal 'something-else', record.friendly_id
+
+      record.name = 'whatever'
+      record.slug = nil
+      record.save!
+      assert_equal 'whatever', record.friendly_id
+    end
+  end
+end
+
 class City < ActiveRecord::Base
   has_many :restaurants
 end